1. Field of the Invention
The present invention relates in general to a digital/analog conversion apparatus having a color palette random access memory (RAM), referred to hereinafter as a palette DAC apparatus, and more particularly to a palette DAC apparatus for multimedia which is capable of inputting color information from two information supply sources, performing a switching operation for the inputted color information to select one of them and outputting the selected color information in the form of digital information as well as analog information.
2. Description of the Prior Art
Recently, techniques in a multimedia field have rapidly been developed to process information more effectively and transfer the processed information innovationally. According to such a trend, in the multimedia field, a technical requirement comes to the fore for the purpose of expressing video color information from a plurality of information supply sources such as, for example, a television (TV), a video cassette recorder (VCR) or a video tape recorder (VTR), a video camera, a compact disk (CD) and the like simultaneously with a video graphics array (VGA) color information of a personal computer (PC) on a single screen.
Conventional techniques for meeting such a technical requirement will hereinafter be described with reference to FIGS. 1 to 4.
Referring to FIG. 1, there is shown a block diagram of a conventional palette DAC apparatus. As shown in this drawing, the conventional palette DAC apparatus comprises latch circuits 1 and 4, a pixel mask register 2, a color palette RAM. 3, digital/analog (D/A) converters (referred to hereinafter as DACs) 5-7, and a microprocessor interface circuit 8. The DACs 5-7 are adapted to output analog red (R), green (G) and blue (B) signals, respectively.
The conventional palette DAC apparatus as shown in FIG. 1 constitutes one of important parts being now used in a VGA card and is well-known as a commercially available single IC chip.
Basically, a finite number of color values are mainly used to constitute a picture on a screen. The finite number of color values are stored into the color palette RAM 3 for a predetermined time period through the microprocessor interface circuit 8 under a control of a microprocessor (not shown) which is provided at the outside of the palette DAC apparatus. The color values are read from the color palette RAM 3 and then outputted through the microprocessor interface circuit 8 to the outside of the palette DAC apparatus, as needed. In this process, the color values in the color palette RAM 3 may be corrected at any time as needed.
A pixel address signal from a display control circuit (not shown) is applied to the palette DAC apparatus for the display of a color picture on the screen. The pixel address signal from the display control circuit addresses a corresponding location of the color palette RAM 3 through the latch circuit 1 and the pixel mask register 2. As addressed, digital color information or the color value from the corresponding location of the color palette RAM 3 is transferred to the three DACs 5-7 through the latch circuit 4. Then, the three DACs 5-7 convert the digital color information into analog color information and output the analog color information to the outside of the palette DAC apparatus.
In this case, an analog color value to be outputted to the outside of the palette DAC apparatus was recorded as a digital color value in a location of the color palette RAM 3 which is recognized by a software operating system or the user. Therefore, for the display, the pixel address signal from the display control circuit is applied to the palette DAC apparatus to address the corresponding location of the color palette RAM 3 in which the digital color value to be displayed was stored. As a result, the digital color value or information from the addressed location of the color palette RAM 3 is transferred to the three DACs 5-7 through the latch circuit 4. Then, the three DACs 5-7 convert the digital color information into three, red, green and blue analog color information and output the analog color information to the outside of the palette DAC apparatus.
The red, green and blue color output values may be expressed by electrical analog numerical values which are specified by an international standard and are directly applied to a cathode ray tube (CRT) for displaying the color information thereon.
The use of the color palette RAM 3 in the above manner is based on a characteristic that colors constituting one frame are not large in number, and has the effect of reducing a size of a video memory which stores video information in the unit of frame therein and is provided at the outside of the palette DAC apparatus. Namely, the video memory may store the red, green and blue color information themselves to express one frame directly as the red, green and blue color information. In this case, the size of the video memory is greater than that in the case of expressing one frame indirectly by storing the color values in the color palette RAM 3 and addressing the corresponding locations of the color palette RAM 3. In the case of expressing one frame indirectly, the size of the video memory can be reduced at a ratio of storing addresses of the color palette RAM 3 to the maximum number with respect to the case of expressing one frame directly.
By the way, in the multimedia field, video signals from original video equipments such as the TV, the VCR (VTR), the video camera, the CD and the like can be viewed on a screen of the PC together with information from the PC, which is regarded as one of functions of the PC. On the contrary, the information from the PC can be viewed on screens of the video equipments together with the video information from the video equipments in a video information processing manner. In such a function, it is preferred to process the signal information in a digital form in order to re-process or store the color information in various forms.
FIGS. 2 to 4 are block diagrams illustrating examples of conventional display circuits using the palette DAC apparatus in FIG. 1, which are capable of performing the above-mentioned function in the multimedia field. Namely, in the conventional display circuits in FIGS. 2 to 4, video color signals from the video media such as the TV, the VCR (VTR), the video camera, the CD and the like can be displayed on a single screen simultaneously with a color signal which is generated from the palette DAC apparatus in response to a pixel address signal from a VGA circuit in the PC.
In FIGS. 2 to 4, the reference numeral 10 designates the palette DAC apparatus in FIG. 1, the reference numeral 11 designates an analog switching circuit, the reference numeral 12 designates a digital/analog (D/A) converter, the reference numeral 13 designates an analog/digital (A/D) converter, and the reference numeral 14 designates a digital switching circuit.
In FIG. 2, the analog switching circuit 11 is adapted to select one of the analog color signal from the palette DAC apparatus 10 and the analog video color signal inputted directly therein. It should be noted that an output value from the analog switching circuit 11 is color information (R, G and B) of a pixel which is a basic unit element of a picture on the screen. Therefore, a switching operation of the analog switching circuit 11 may be performed so that the video color signal from one of the video media such as the TV, the VCR (VTR), the video camera, the CD and the like can be displayed on one area of the screen and the color signal from the palette DAC apparatus 10 corresponding to the pixel address signal from the VGA circuit in the PC can be displayed on the other area of the screen. That is, two types of color information can be displayed simultaneously on the single screen.
In this case, each of the color signal information or analog R, G and B color signal values to the CRT has a peak amplitude value of 714 mV or 1.0 V which is specified by the international standard, and a resolution which is determined according to the number of bits for the process of the color signal information in the digital form. In other words, an analog color amplitude is divided in equal parts of the number which is obtained by taking the number of the bits as an exponent for 2. For example, the analog color amplitude is divided in 64 equal parts if the number of the bits is 6 and in 256 equal parts if the number of the bits is 8.
On the other hand, in FIG. 2, the switching operation of the analog switching circuit 11 is performed under an analog condition, resulting in generation of a noise. The generation of the noise resulting from the switching operation exerts a bad influence on the final analog color signal information or the output value from the analog switching circuit 11. Namely, the generation of the noise resulting from the switching operation may result in a degradation in the resolution since a minimum amplitude value of the analog color signal which is determined according to the number of the bits used is very small.
Also in the multimedia field, for the convenience of the user, there is provided a function of controlling freely a size of a window of the screen in which the video signal is expressed. In order to control freely the size of the window of the screen, it is preferred to process the video color signal in the digital form. Only in the case where the video color signal from the video medium to be expressed on the screen is stored in the digital form in a memory, it can be outputted from memory locations corresponding to a positive number multiple of an initial address or from the same memory location addressed by the positive number multiple times so that it can readily be adaptive to the size of the window of the screen.
This means that the video color signal from the video medium must be in the digital form before reaching the analog switching circuit 11. Namely, the resolution of the video color signal from the video medium can be controlled suitably for the size of the window of the screen by using an interpixel interpolation method or deleting adjacent pixel information according to a requirement of the user. In this case, the process can readily be performed only when the video color signal from the video medium is in the digital form. For this reason, the conventional construction in FIG. 2 has a disadvantage in that it is difficult to control the resolution of the video color signal from the video medium suitably for the size of the window of the screen.
In FIGS. 3 and 4, digital video color signal information from the video medium and the analog color signal information from the palette DAC apparatus 10 are processed at the outside of the palette DAC apparatus 10 in such a manner that they are selectively applied to the CRT.
In FIG. 3, the digital/analog converter 12 is adapted to convert the digital video color signal from the video medium into an analog color signal. Similarly to that in FIG. 2, the analog switching circuit 11 selects one of the analog color signal from the digital/analog converter 12 and the analog color signal which is generated from the palette DAC apparatus 10 in response to the pixel address signal from the VGA circuit in the PC. As a result of the switching operation of the analog switching circuit 11 at the interval as mentioned above, two types of color information can be displayed simultaneously on the single screen However, similarly to that in FIG. 2, the conventional construction in FIG. 3 has a disadvantage in that the generation of the noise resulting from the switching operation of the analog switching circuit 11 exerts the bad influence on the analog R, G and B color signal from the analog switching circuit 11.
In FIG. 4, the analog/digital converter 13 is adapted to convert the analog R, G and B color signal which is generated from the palette DAC apparatus 10 in response to the pixel address signal from the VGA circuit in the PC into a digital color signal. The digital switching circuit 14 is adapted to select one of the digital video color signal from the video medium and the digital color signal from the analog/digital converter 13. The use of the digital switching circuit 14 makes it possible to display two types of color information simultaneously on the single screen, in the same manner as that of the analog switching circuit 11. The output of the digital switching circuit 14 is converted by the digital/analog converter 12 into an analog color signal to be displayed on the screen of the CRT.
The analog/digital converter 13 must have a resolution higher than that of the analog signal from the palette DAC apparatus 10 because the analog signal is relatively sensitive to the noise as compared with the digital signal. In the case where the color signal to be displayed on the screen is intended to be recognized as a successively moved video, there is required an operating speed which is in proportion to a value obtained by multiplying the number of frames per a second by the number of pixels constituting one frame. For this reason, in some cases, there may be required high speed-convertible analog/digital converter and digital/analog converter. However, it is common that the analog/digital converter has a conversion speed lower than that of the digital/analog converter because of its internal circuitry. As a result, the operating speed of the entire circuit is limited to the conversion speed of the analog/digital converter.